1. Technical Field
The presently disclosed technologies are directed to a system and method for reducing the magnitude of the electrostatic field as a printing media substrate is transported underneath print heads. The system and method described herein use active biased electrodes to reduce the magnitude of the electrostatic field on a printing media substrate and decrease potential print quality defects.
2. Brief Discussion of Related Art
In order to ensure good print quality in direct to paper (“DTP”) ink jet printing systems, the media must be held extremely flat in the print zone. Some proposed methods for achieving this use electrostatic tacking of the media substrate to a moving transport belt that is held flat against a conductive platen in the imaging zones. An undesirable side effect of electrostatic tacking of media is the creation of a high electric field between the media and the imaging heads (also referred to herein as print heads). As the media travels in the printing zone, the high electrostatic field can affect the ink jetting, which results in print quality defects.
FIG. 1 depicts an exemplary prior art printing system. The media substrate (MS) is transported onto the hold-down transport using a traditional nip based registration transport with nip releases. As soon as the lead edge of the media is acquired by the hold-down transport, the registration nips are released. A vacuum belt transport is used to acquire the media substrate (MS) for the print zone transport (PZT).
FIG. 2 depicts an alternate prior art method for media acquisition wherein electrostatic forces are used to tack the media substrate (MS), e.g., paper, onto a transport belt (TB) that is supported by a metal conductive belt platen support (BS) underneath the print zone. The figure shows an exemplary media tacking method which is well known in the state of the art. The transport belt (TB) can be fabricated from relatively insulating (i.e., volume resistivity typically greater than 1012 ohm-cm) material. Alternatively, the transport belt (TB) can include layers of semi-conductive material if the topmost layer is made from relatively insulating material. If semi-conductive layers are included in the transport belt, the quantity “volume resistivity in the lateral or cross direction divided by the thickness of the layer” or “sheet resistivity” is typically above 108 ohms/square for any such included layers. The basic belt transport system includes a drive roll (D), tensioning roll (T) and steering roll (S). The transport belt material may be an insulator or a semiconductor. The basic media tacking is shown in the dashed box upstream of the print heads (PH). Two rolls (1 and 2) are used. Roll 1 is on top of the belt/media and roll 2 is below the belt (TB). A high voltage is supplied across roll 1 and roll 2 to produce tacking charges that adhere the media substrate (MS) to the transport belt (TB). An optional blade (B) (shown upstream of the rollers) can be used to enhance tacking by forcing the paper against the belt just prior to the rollers. Biased roller charging is generally preferred but optionally, many other media charging means that are well known in the art can be employed in place of the biased roller pair shown. For the purposes of this disclosure, biased roller charging is inclusive of all of the various charging means that can be used.
Either roll 1 or roll 2 may be grounded, but there is a preference that roller 1 be grounded. This preference is mainly due to media tacking problems that can occur with very moist, low resistivity media due to conductive loss of charge on the media caused by lateral conduction of charge on the media to grounded conductive elements such as lead-in baffles that contact the media prior to the charging rollers. As is known in the art, this loss of charge can be solved by applying and/or inducing high voltages on the conductive lead-in baffles, but this adds some cost to supply the voltages. It requires that the baffles be well isolated from ground, and it also requires precautions to prevent machine operators from contacting the baffles during machine operation. Grounding the top roll avoids the need for any of this.
Since the top most surface of the transport belt is relatively insulating, charge can build up on the belt with each cycle of the belt. After a number of cycles, this can prevent adequate tacking of the media to the transport belt in the media charging zone. To avoid this, the charge state of the belt should be stabilized prior to the rollers 1 and 2 charging zone. In particular, the potential VS above the belt at a grounded roller just prior to the media charging zone (such as roller S in FIG. 2) should be kept to a relatively stable and controlled value for each belt cycle. The cyclic stabilization of the belt charge state can be accomplished by providing a charging device that faces one of the grounded rollers below the transport belt prior to the media charging zone. For example, a corotron charging device (not shown) at the roller T position in FIG. 2.
Media, tacked by electrostatic tacking methods, almost always produce an electric field. When the media travels through the print zone, the high electric field between the media and the print heads due to the electrostatic tacking can interact with the ink ejection. This can frequently produce print quality defects. Accordingly, it is desirable to reduce the magnitude of the electric field when the media passes the print heads in order to mitigate or eliminate print quality defects.